Vibration damping is a common need in many mechanical systems where undesired resonances may be excited by normal perturbations. The suspension system in an automobile, for example, will exhibit large unwanted oscillations in response to road irregularities unless properly damped. A typical suspension system consists of large springs, coupled between the chassis and axles, which provide compliance as bumps are encountered. This allows the mass of the chassis to accelerate slowly in response to impulses. Shock absorbers, which produce forces opposing the velocity of compression or elongation of the springs, are employed to provide appropriate damping and inhibit oscillations.
The resonant frequency and size of automobile suspension systems allow the construction of shock absorbers which are often relatively complex mechanical contrivances (containing fluids, dynamic seals, etc.). Many systems, however, are better suited to the application of materials which inherently provide damping for oscillatory motions. Particularly where low amplitude and/or high frequency oscillations occur, it is desirable to directly couple materials with good damping properties to vibrating components.
Materials used for vibration damping should exhibit large viscous losses in response to deformation. These losses are typically quantified in terms of either dynamic Young's moduli or dynamic shear moduli. In either case, the dynamic storage modulus, by definition, is proportional to the amplitude of the stress which results in response to a sinusoidal strain (where the strain may be either shear or elongational depending on whether shear or Young's modulus is desired respectively). Similarly, the loss modulus is, by definition, proportional to the amplitude of the stress which results in response to the application of a sinusoidal strain rate. The ratio of dynamic shear loss modulus to dynamic shear storage modulus, or dynamic Young's loss modulus to dynamic Young's storage modulus, at a particular oscillation frequency, is commonly referred to as tan .delta.. The magnitude of the loss modulus in a material quantifies its viscous-like resistance to deformation while tan .delta. quantifies the relative magnitude of this resistance to elastic response (McCrum et al., Anelastic and Dielectric in Polymeric Materials, John Wiley and Sons 1967).
Although damping performance may be characterized both in terms of tensile/compressive moduli and shear moduli, these two sets of moduli have a well defined relationship to one another. In particular, dynamic shear moduli are related to dynamic Young's moduli through a relationship which depends upon the Poisson ratio, which may also be frequency dependent. Because the relationship between Young's and shear moduli at a particular frequency is constrained to fall within relatively narrow limits, the two sets of moduli nearly always track each other in a monotonic fashion. It is therefore practical to quantify damping performance in terms of either dynamic shear or Young's moduli. For the sake of clarity, dynamic Young's moduli will be used for all discussion, analysis, and characterization to follow herein. In addition, the tan .delta. used subsequently herein will be defined as the ratio of the dynamic Young's loss modulus to the dynamic Young's storage modulus, at a given frequency, and all references to dynamic loss and storage moduli will refer to Young's Moduli.
The specific properties a damping material must possess are dictated by the constraints of typical applications. Damping materials, due to their unique mechanical properties, are not commonly used as structural materials but are incorporated into a system in combination with stiffer structural elements. For this reason, it is desirable to use the minimum possible amount of damping material in a given system such that the cost, volume, or mass of the damping component is minimized. This is particularly true in space, aircraft, or automotive applications (where weight is an important constraint) and in situations where the addition of damping components adds undesired mass affecting system response/performance (damping of vibrations in disk drive read/write heads, for example, requires low mass components which will not adversely affect the momentum of the head). For this reason, a purpose of this invention is to provide materials with the highest possible loss moduli, relative to the storage moduli of other materials comprising the system.
Constraints also exist on the ratio of dynamic loss moduli to dynamic storage moduli, tan .delta.. Extremely stiff materials, even with large dynamic loss moduli, will be dominated by elastic effects and behave like springs. Large dynamic loss modulus, therefore, is not sufficient to ensure good damping characteristics at a particular frequency. It is also necessary that the ratio of dynamic loss to dynamic storage modulus be as large as possible, thus insuring sufficient loss relative to purely elastic behavior.
A number of approaches have been taken to achieve material properties sufficient for damping purposes. Specialized formulations of crosslinking polymers have been developed which exhibit damping in specific applications. Epoxy formulations have been developed for damping vibrations in magnetic recording heads, as disclosed in U.S. Pat. No. 5,270,888, and for damping in cutterhead assemblies used for the manufacture of high-density information discs, as disclosed in U.S. Pat. No. 4,488,282. Acrylic co-polymers for damping are commercially available and, in sheeting form, and have been sandwich bonded to steel plates using flexible magnetic materials as disclosed in U.S. Pat. 3,817,356. Silicone chemistries have been developed for vane damping, as disclosed in U.S. Pat. No. 5,434,214. In addition, networks of polyurethane-epoxy formulations have been applied to acoustic damping, as disclosed in U.S. Pat. No. 5,331,062. Formulations of crosslinked polymers have been achieved with dynamic loss modulus approaching 10.sup.9 dyne/cm.sup.2 Hz at 20.degree. C., in the frequency band from 0.1 Hz to 10.sup.5, and tan .delta. sufficient to provide reasonable performance in typical damping applications. In addition, these formulations have been engineered to provide enough resistance to cold flow to allow application in the examples outlined above.
However, improved damping characteristics are commonly achieved at the expense of other desired properties. Specifically, manipulation of molecular weight, degree of crosslinking, etc., to improve internal losses, typically leads to undesirable creep resistance, since internal loss in these systems is highly correlated with cold flow. Increase of damping properties beyond a given point in known chemistries, therefore, often leads to materials unable to maintain shape, even under the influence of gravity. Since such characteristics are highly undesirable in practical application, this places serious fundamental constraints on the achievement of improved damping characteristics in practical forms. For this reason, in all of the prior art listed above, fundamental limitations exist on damping performance achievable in articles with good mechanical stability.
To significantly advance the art, therefore, some mechanism for producing high damping performance, combined with mechanical stability, is needed. It is a purpose of the present invention to provide an unexpected means of creating materials with extremely high damping performance, and exceptional resistance to cold flow. As the detailed description below will describe, such stabilization and damping is achieved by the composite of the instant invention.
One example of prior art in the general area of damping composites is the foam construction disclosed by Teroson GmbH in U.S. Pat. No. 4,374,172, wherein materials are incorporated within foams, in varying concentrations spatially, for the purpose of tailoring damping performance across a given system. However, Teroson makes no mention of stabilization of materials which exhibit cold flow. In fact, their trivial binding of a damping material into a foam would not guarantee mechanical stabilization or necessarily provide a high damping performance composite. Serious thought must be given to materials selection because performance will be limited if, for example the foams are too compliant or fail to be sufficiently loaded. No such teaching is provided by the Teroson patent.
Other art relates to the incorporation of fibers into damping materials, or the lamination of such materials between high strength layers of material. High tensile strength fibers have been incorporated into specially formulated damping resins to yield materials for high strength flexible beams with reasonable damping characteristics, as disclosed in U.S. Pat. No. 4,304,694. In addition, damping materials have been laminated between high strength polymer films, as disclosed in U.S. Pat. No. 5,368,916, and sandwiched between very high strength fiber reinforced materials, as disclosed in U.S. Pat. No. 5,487,928, to attempt to achieve higher strength damping composites.
Although fiber composites and laminate/sandwich construction can improve the tensile strength of mechanically stable damping materials, they are still markedly deficient in their ability to significantly improve the cold flow of the composite. In situations where the base material exhibits creep, relative motion of unconnected fibers within the composite is possible. Similarly, lamination of unstable materials between layers of stronger components allows relative motion of these layers and consequent instability. Such constructs, therefore, are themselves unstable if the base material flows over time. This instability places limitations on these composites nearly identical to those of the damping materials upon which they are based. For this reason, U.S. Pat. Nos. 4,304,694, 5,368,916, and 5,487,928 do not teach true stabilization of high loss, mechanically unstable, damping materials. It would be desirable if mechanisms could be found to provide connectivity between the high strength components of these constructs, combined with a mechanism for locking an unstable damping material within the structure. Such a technology would be remarkably novel and potentially yield materials with damping performance never achieved in a mechanically stable form.
Considerable effort has also been devoted to the application of existing damping materials in geometries which optimize overall damping performance (Sun et al., Vibration Damping of Structural Elements, Prentice Hall, 1995). As is well known, the effective performance of a given damping material can be significantly enhanced through the application of constraining layers which induce shear deformations in the damping material as vibration occurs. However, damping limitations are still imposed by the fundamental dynamic losses of the damping material employed. Given a fixed constrained layer geometry, for example, performance improvement is only achievable through the use of damping materials with greater losses.
Since application of constrained layer or other geometrical enhancement, will have limitations set by existing damping characteristics, materials are needed with improved dynamic loss moduli and sufficient tan .delta. across the various frequency bands important for specific applications. In addition, materials sufficiently versatile to allow tailoring of response to the vibrational resonances present in specific applications are needed. Finally, the desired damping properties must be achieved without unacceptable degradation in other important physical properties.
In the art cited above, the viscoelastic damping materials employed were specifically tailored to possess good resistance to cold flow. For this reason, the achievement of damping performance, i.e., the lessening of the amplitude of waves or oscillations, has been, heretofore, limited by the trade off with mechanical stability with respect to cold flow. Clearly, a vehicle for extending these technologies to allow the use of damping materials which possess much higher loss properties but which, inherently, exhibit unwanted cold flow, would provide unique utility and value.